Internet IP Addressing

Behind every URL (website server) and every computer or other device connected to the Internet is a number that uniquely identifies that device. This unique identifier is called an IP address. These addresses are the key components of the routing schemes used over the Internet. For example, if you are downloading a data sheet from http://www.yahoo.com to your computer, the header of the packets composing the document includes both the host address (in this case, the IP address of the Yahoo public server) and the destination address (your PC).

Each IP address is a 32-bit number, which means address combinations number 4.3 billion. You must allocate these addresses in such a way that balances the need for administrative and routing efficiency and the need to retain as many usable addresses as possible.

The most common notation for describing an IP address is dotted decimal. Dotted decimal breaks up a 32-bit binary number into four 8-bit numbers (represented in decimal form). Each section is separated by a period, which aids in the organizational scheme. For example, you can represent the binary address 1010100000001011001000101110 in dotted decimal as 10.128.178.46.

Logical Versus Physical
Media Access Control (MAC) addresses are considered physical addresses because the manufacturer permanently assigns them to pieces of hardware. (They cannot be reassigned.)

A network administrator assigns IP addresses, which only have meaning in a Transmission Control Protocol/Internet Protocol (TCP/IP) network. Used solely for routing purposes, these addresses can be reassigned.

Rather than assign numbers at random to various endpoints (which would be extremely difficult to manage), every company and organization on the Internet has a block of address numbers to use. A two-part addressing scheme identifies a network and a host. This two-part scheme dictates the following:

• All the endpoints within a network share the network number.
• The remaining bits identify each host within that network.

In the following example, the first two octets (128.10) identify a company with an Internet presence. (It’s the address of the router that accesses the Internet.) All computers and servers within the company’s network share the same network address. The next two octets identify a specific endpoint (computer, server, printer, etc.). In this example, the company has 65,536 addresses it can assign (16 bits or 2¹⁶).

Address Classes
When the IP address scheme was developed, only the first octet identified the network portion of the address. At the time, scientists assumed that 254 networks would be more than enough to cover the research groups and universities using this protocol. As usage grew, however, it became clear that the Internet would need more network designations (each with fewer hosts). This issue led to the development of address classes.

Addresses fit into five classes (Aâ€'E). Classes A, B, and C are the most common. Class A addresses have 8 network bits and 24 host bits. Class C addresses have 24 network bits and 8 host bits. This scheme was based on the assumption that the Internet would have more small networks (each with fewer endpoints) than large networks. Class D is used for multicast, and Class E is reserved for research. The following table outlines the three main classes. Note that the Class A address starting with 127 is reserved.

You can derive the total number of available hosts on a network by using the formula 2’-2, where n is the number of host bits. The -2 accounts for an octet with all Os (which identifies the network) and all Is (which is a broadcast address).